Circuit breaker



April 1959 H. M. STEVEN ETAL 2,884,497

CIRCUIT BREAKER Filed Aug. 22, 195'? 3 Sheets-Sheet 1 INVENTORS HARRY M.STEVEN, RAYMOND W. MARSHALL ATTORNEY Abril 28, 1959 Filed Aug. 22, 1957H. MIS'FEVEN EI'AL CIRCUIT BREAKER 3 Sheets-Skieet 2 INVENTORS HARRY M.STEVEN,

RAYMoNDW. MARSHALL April 28, 1959 H. M. STEVEN ETAL 12,884,497

CIRCUIT BREAKER Filed Aug. 22, 1957 3 Sheets-Sheet 3 INVENTORS HARRY M.STEVEN, RAYMoNp W. MARSHALL KMJ C 7 ATTORNEY United States PatentCIRCUIT BREAKER Harry M. Steven, Schenectady, and Raymond W. Marshall,Scotia, N.Y., assignors to General Electric Company, a corporation ofNew York The present invention relates to electric circuit breakers andparticularly to circuit breakers of the type including thermal andmagnetic tripping means.

Circuit breakers of the type referred to ordinarily include thermaltripping means for causing automatic opening on low and intermediatevalue overload currents. In addition, magnetic or instantaneous actingtripping means is included for causing automatic opening in response tooverload currents of short circuit magnitude, that is, currents in theneighborhood of five or more times the circuit breakers nominal currentrating. The particular value at which thermal tripping occurs is pre-setwhen the breaker is manufactured, and, for safety reasons, should not bechanged during use. In the case of the magnetic or instantaneoustripping, how ever, it is desirable that the maximum and minimuminstantaneous tripping points be set by the manufacturer, but that acertain latitude of adjustment be permitted the user. Apparatus forpermitting such setting of maximum and minimum tripping points andadjustment within this range by the user have in the past beenrelatively complicated. In addition, such adjustments have not beenconvenient for the user to use, requiring the use of a screwdriver, etc.It has also been difiicult, with prior art constructions, to set suchadjustments accurately and to observe or read the settings.

It is an object of the present invention to provide an electric circuitbreaker including magnetic tripping means which is relatively simple andwhich can be adjusted with a high degree of accuracy.

It is another object of the present invention to provide an electriccircuit breaker including magnetic tripping means which is readilyaccessible for manual operation without the use of tools.

It is another object of our invention to provide an electric circuitbreaker including magnetic tripping means having adjusting means thesetting of which can be readily determined by observation.

In accordance with the present invention, an electric circuit breakertrip unit is provided including a stationary magnet field piece and apivotally supported armature movable toward the field piece to causetripping. The armature includes an extension which is adapted to supportone end of a biasing spring biasing the armature to its open orunattracted position. The opposite end of the biasing spring issupported on a pivotally mounted adjusting lever. The adjusting lever,in turn, is supported in such a manner that rotation thereof carries theline of action of the biasing spring closer to or farther away from thepivotal axis of the armature. This varies the effective biasing force ofthe biasing spring on the armature, substantially without change in itslength, thereby varying the amount of 'magnetic pull and the amount ofcurrent needed to actuate it.

In accordance with another aspect of the present invention, the tripunit is provided with an insulating housing and the adjusting lever hasits outer end brought through a specially formed'slot in the wall of thehousing to be accessible for manual movement in such a manner that theslot does not altord a straight-in opening to the interior of the tripunit.

In accordance with another aspect of the present invention, the biasingspring is attached to the adjusting lever by an adjustable connector.

This connector is accessible through the trip unit housing even afterassembly of the unit, and serves to adjust the length and pull of thebiasing spring. This establishes the maximum and minimum tripping pointsof the magnetic tripping means.

Additional objects and aspects of the invention will be pointed out inthe following detailed description and illustrated in the accompanyingdrawings and the scope of the invention will be particularly pointed outin the appended claims.

In the drawings:

Figure 1 is a side elevation view partly in section, of an electriccircuit breaker incorporating the invention;

Figure 2 is a perspective view of a circuit breaker trip unitconstructed in accordance with the invention;

Figure 3 is a side elevation view of a portion of the tripping mechanismof the circuit breaker trip unit of Figure 2;

Figure 4 is a fragmentary view of a portion of the mechanism of Figure 3with the bimetallic strip shown in a heated or defective condition;

Figure 5 is a sectional view taken along the lines 55 of Figure 7;

Figure 6 is a sectional elevation view taken along the lines 66 ofFigure 7;

Figure 7 is a side elevation view of the trip unit of Figure 2;

Figure 8 is a sectional view of a portion of the trip unit of Figure 2taken generally on the line 8-8 of Figure 7;

Figure 9 is an exploded view of the interior parts of the circuitbreaker trip unit of Figure 2, with certain parts removed for clarity.

In the drawings, the invention is shown as embodied in a three-poleelectric circuit breaker comprising an insulating outer or main casinghaving a base 10 and a cover 11, and including three load terminals 12(only one shown) at one end. A removable trip unit 23 is mounted withinthe main enclosure adjacent the load terminals 12 and is electricallyconnected to the load terminals 12 by means of conductor straps 24, andto the circuit breaker contacts (not shown) by conductors 22.

The circuit breaker includes an operating mechanism, not shown, whichmay be of conventional form, and which includes a handle member 32 formoving the contacts between open and closed. circuit positions manually,and a releasable member 31 which is movable to cause automatic openingof the circuit breaker contacts upon the occurrence of predeterminedcurrent conditions. The releasable member 31 is normally retained in apredetermined latched position and is released upon the occurrence ofpredetermined current conditions by means of the trip unit 23. A type ofoperating mechanism suitable for use with the present invention is shownin application Serial Number 679,607, filed August 22, 1957, by E. B.Judd, et al., assigned to the same assignee as the present invention.

The end of the releasable member 31 is formed to act as a latchingelement and to engage an intermediate latch member 34 which is pivotallyand slidably supported on a pin 35 carried by the mechanism frame, notshown. A biasing spring 34' constantly urges the intermediate latch 34in a clockwise direction as viewed in Figure 3. By reason of thesubstantially greater force exerted on it by the releasable member 31,however, the intermediate .and slightly beyond the normal off position.

latch member 34 is biased in a counterclockwise direction when thebreaker is latched as shown.

The trip unit 23 includes an insulating housing, to be more fullydescribed, and alatch member. 36 adapted normally .to prevent theintermediate latch member; 34 from rotating in acounterclockwise-direction. The latch member36 is pivotally supported onthe tripunit v23 on a pivot pin 37 extending through aligned openingsina boss 38 molded integral with the housing of the trip unit.

The latch member 36 carries a latch roller 39 supported on a pin 40extending between opposed spaced 3 apart portions of the latch member36. The intermediate latch member 34 is provided with a cam surface 41which, when the breaker is latched, .bears against the roller 39 andbiases the latch member 36 in a counterclockwise .direction, that is tothe left as viewed in Figure 3.

The latch member36 is prevented from moving to the left by the action ofa pair of toggle links 42 and 43 which are interconnected between arelatively stationarypivot pin 44 carried by the trip unit 23 and thepivot pin 40 carried by the latchmember 36. The toggle links 42 and 43are interconnected and joined by a knee pivot pin 45. The toggle linkageincluding the links 42 and 43, is biased in an upwardly oroversetdirectionby means of a spring 46 which surrounds a portion of the pin 40and has its opposite ends engaging the toggle link 42 and the latchmember 36, respectively. The bias of the spring '46 is such as to urgethe knee pivot pin 45 upwardly as. viewed in Figure 3; tending to causecollapse upwardly of the toggle linkage 42, 43 and therefore movementofthe latch 1 member 36 counterclockwise. as viewed in this Figure 3.

Movement of the knee of the toggle linkage I42, 43 upwardly is limitedbyits engagement with. the toggle adjusting screw 47 carried by anintegral boss 43 on the trip bar 49. The trip bar 49 is, in turn,prevented from rotating in a counterclockwise direction by itsengagement with a trip .bar adjusting screw 50. The parts aretherefore.retained normallyin the condition shown in Figure 3 and in Figure l.

The trip bar 49 is pivotally supported in the trip unit housing on anaxis coaxial with the pin 44. Clockwise rotation of the trip bar 49 bythe current responsive. means in a manner to be described :causes thetoggle adjusting screw 47 to move the knee of the toggle to and throughthe dead center position until the toggle collapses downwardly. At' thesame time, thelatch 36 moves to the left, carrying the latch roller 39,and allowing theintermediate latch member 34 to rotate-counterclockwiseand to release the member31, with consequent openingof the contacts.

Following such release, the intermediate. latch is urged clockwise byits bias spring 34', against a stop portion of the frame, not shown.This permits the trip toggle'4243 to resetitself due to the action ofthe bias spring 46, these parts returning, aftercooling of thebimetallic strip, substantially to thecondition'of Figure 3.

In order to relatch the breaker following atripping operation, .thehandle 32 is moved manually to the left to During this movement, aportion of the handle engages the carrier 31 and rotates itcounterclockwise. 'When the latch tip of the carrier-31 engages thesurface 345 of the intermediate latch 34, further counterclockwisemovement of the carrier causes theintermediate latch34 to move to the.left,.as.viewed, with a combined pivoting andsliding action, aspermitted by the elongated pivot notch 34N. The ends of thebias spring34'also are adapted to slide along the inner surface f the portion 348to permit this sliding retracting action. The latch tip of thecarrier'31 finally clears the latch surface of the intermediate. latch,

. andthe. intermediate latch is returned to its normal position by theaction of the spring 34', thereby relatching the breaker.

.Certain aspects of the trip 'unit mechanism disclosed herein'form apartof the subjectmatter disclosed'and brating screw '52- at its'oppositeend. In order to prevent accidental alteration'of'the calibration suchas by vibration," the. calibrating screw is" mounted in a'lock-nut 53which is, in turn, carriedby the bimetallic strip 51.

In order to heat the bimetallic strip 51 in accordance with currentpassing through the circuit breaker, an intermediate portion 54 of theconductor is provided between the terminals 22 and 24 comprising aheater portion. As will-be observed from Figure 9, the heater portion 54is of reduced cross section thereby-increasing the resistance of this.portiom of the conductor and concentrating the r able means suchas byscrews 74 and 75.

heating therein. Current does not pass through thebimetallicstripmember. .Instead, the bimetallic member is mountedclosely adjacent and preferably against the heater portion 54 of theconductor. .The terminals 22 and 24, together with. the intermediateheater portion 54, comprise a single continuous conductor .56 ofrelatively rigid nonlaminatedmaterial which is fastened to the base bysuit- The terminal portions 22 and 24 are therefore adapted to .serve asmounting supports for the trip unit, and, since theyv are relativelyheavyand inflexible, are. adapted to prevent movement onmisalignment ofthetrip unit with respect to, the circuit breaker mechanism. which mightcause a change in calibration of the breaker.

-Magnetic orinstantaneous tripping action is provided by means 10farelatively stationary magnet .55 surrounding the conductor56sandmounted on the trip unit housing by suitable means such as by screws 57.The magnet 55 is generally U-shaped and surrounds the heater portion'54, whereby the current passing through the heater portion also servesto create a magnetic field in the magnet 55. Magnetic armatures58 areprovided, one for each pole, pivotallysupported with respect to themagnet member; 0n the. same pivotal axis as the trip bar 49. Each ofthearmatures :58 includes :upwardly bent portions '58 having alignedholes therein adapted to receive one of the" pivot pins 44.

' Upon the.occurrenceiofexcessive overloads or short circuit currents,the magnet'SS attracts the armature 58 causing it to strike a projectionon the trip bar49, rotating the trip bat- 49 clockwise, collapsing thetoggle linkage and tripping the cireuit'breaker.

"Thearmature 58 is biased at all times in a counterclockwise directionagainst an adjustable stop screw 59 (Figure 6). For'thispurpose, atension spring 60 is provided having-one-end thereof connected-to thearmature v58 at the'upwardlybent portions 58, and the other endconnected to an adjustinglever 61. The adjusting lever 61 is pivotallysupported in an elongated recess or notch 61N inthe trip unit housing,being held there by the bias of. the spring60. As the adjusting lever 61is moved from itsupper to its lower position, as indicated in Fig-.ure.6,. the line ofactionof .theadjusting spring 60 is moved: closer-to thepivot point of the armature 58, and

vice versa. This changesthe'lefiective-bias of the spring .60u0nf'the.magneticarmature-and-varies the amount of ":current5required tocausemagnetic' actuation.

.The trip.. barr49- is :provided with spaced deeply cut notches 49'extending therewithinto a point past the-axial center- 0f the pivot'pins44.for the purpose of providing clearance for the adjustinglever61,whereby the levers 61 I 'may move the-springs 60rto a positionsubstantially in linewith' the pivotal axis of the -tn'pbar 49 whenin'the fully depressed condition such for instance as shown in dottedlines in Figure 6.

A fiat spring member 62 is attached to the lever 61 and has its outerend return-bent. This provides a rounded cam surface adapted to rideagainst the cam surfaces 63 formed in the trip unit housing. The actionof this spring 62 in moving over the cam surfaces 63 aifords astep-bystep adjusting action for the lever 61.

Adjustment of the maximum and minimum magnetic tripping values isachieved by an adjustable mounting of the spring 60 on the lever 61.This adjustable mounting includes a short mounting lever 64 pivoted onthe lever 61 at 65, and carrying the end of the spring 60 at its outerend. The mounting lever 64 includes an offset portion 66 which bearsagainst an adjusting screw 67 carried by the main adjusting lever 61. Asthe adjusting screw 67 is turned inwardly, the lever 64 is rotatedclockwise as viewed in Figure 6, stretching the tension spring 60, andincreasing the bias on the magnetic armature 58. Thus the length of thespring 60 is varied without moving the lever 61.

The trip unit housing includes two mating molded parts 68 and 68, thepart 68 being larger and serving as a base for supporting the variousparts of the trip mechanism. The part 68' serves as a cover and alsosupports some of the parts. The two portions 68 and 68' of the trip unit23 are adapted to be fastened together by suitable means such as bybolts and nuts 88, arranged to be insulated and made substantiallynon-removable by filling over the ends thereof with a suitable materialsuch as a potting compound or cement. Other fastening means such asspun-over tubular rivets may of course be used instead of bolts.

In order to enclose the adjusting means for the armatures 58 withoutunduly enlarging the trip unit housing, the cover member 68 is providedwith projecting portions or chambers 76 at corresponding points.

The trip unit casing part 68 includes an arcuate boss 69 for each poleof the circuit breaker, each arcuate boss having an elongated slot 70through which the outer portion of the adjusting lever 61 projects. Eachslot 70 also preferably includes a shield portion 71 at right angles tothe general direction of the adjusting lever 61, the adjusting lever 61having an offset integral portion 72 passing through the slot. Each ofthe slots 70 therefore does not afford a direct opening to the interiorof the trip unit.

Initial assembly of the adjusting lever 61 into the slot 70 isaccomplished by providing each slot 70 with a portion 73 at one endwherein the shield 71 is omitted to admit the outer end of the adjustinglever 61 including the offset portion 72.

Each of the magnets 55 preferably has two auxiliary pole members 77,each comprising an elongated strip of magnetic material projectingbeyond the pole faces of the magnet 55 and having its outward endupwardly bent to extend close-1y adjacent to the armature 58 when theyare in their unattracted position. The auxiliary pole members 77maintain a relatively small backward pull on the armatures 58 duringnormal currents and minor overload currents. On the occurrence ofrelatively large overcurents, however, the intermediate portions of theauxiliary pole member 77 become saturated and the pull of the main poleportions of the magnet 55 greatly exceeds the backward pull of theauxiliary poles 77. The net effect of this action is to cause thearmatures 58 to move to closed position with what may be termed a snapaction. The auxiliary pole members 77 may be and preferably are,

omitted in forms of the invention used for lower ampere rated circuits,such for instance at 125-300 amperes.

The trip unit housing members 68 and 68' have matching cut-away portionswhich provide openings or windows in the top wall thereof adapted topermit access to the adjusting screws 67. Following adjustment of thescrews 67, the apertures are closed by relatively thin fiber insulating'members 78, which are snapped in place, being trapped therein undertapered ledge retaining portions 78'. The trip unit base 68 also hasapertures 79 which are aligned with calibrating screws 52, and throughwhich the calibrating screws 52 may be adjusted from outside the unitcasing after the casing is completely assembled.

In order to facilitate testing of the mechanical operation of the tripunit when installed in a circuit breaker, means is provided for trippingthe trip unit manually when desired. For this purpose, a plunger 80 ismounted in a recess in the trip unit base 68 and has a reduced portionextending through the trip unit wall, with the inner end adjacent thetrip bar 49. A biasing spring 80', under the head of the plunger 80,biases it outwardly away from the trip bar, outward travel being limitedby a retaining clip or washer on the inner end of the plunger. An inwardforce on the plunger 80 against the force of a bias spring 80, causesthe inner end of the plunger to engage the trip bar 49 rotating it in atripping direction.

The trip unit cover 68 supports the parts relating to the trippingtoggle and also supports a bracket member 81 (Figure 2) adapted tointerengage with a portion of the side frames of the mechanism proper,to prevent any relative movement between the two members.

Certain aspects of the circuit breaker and trip unit enclosureconstruction disclosed herein form a part of the subject matterdisclosed and claimed in application Serial Number 679,589, filed August22, 1957, by R. W. Dunham and C. L. Jencks and assigned to the sameassignee as the present invention.

It Will be observed that the trip mechanism of the trip unit 23,including the thermal and magnetic tripping elements have been providedwith a number of independently manipulatable adjustments as indicatedparticularly in Figure 3. Adjustment and calibration of the circuitbreaker is carried out using these adjustments, in the following manner.Initially, before the trip unit is mounted in the circuit breaker, alladjusting screws are retracted. With no current passing through thebreaker, the armature adjusting screw 59 is turned in until the armaturetouches magnet 55 (closed position). The screw 59 is then retracted apredetermined small amount such as one turn, establishing a firstposition of the armature short of the closed position. With the parts inthis condition, the toggle adjusting screw 47 is turned inwardlysufiiciently to cause the toggle links 42, 43 to move to collapsedposition. This assures that when the armature has reached its firstposition short of closing, the trip toggle will have been broken.Following this, the screw 59 is then turned outwardly an additionalpredetermined amount such as 3 /2 turns to establish a desired initialair gap, set in accordance with the current at which tripping isdesired. The trip bar adjusting screw 50 is then turned in sufficientlyto contact the trip bar 49, thereby establishing the normal position ofthe trip bar. Following the preliminary adjustments, the trip unit ismounted Within the circuit breaker proper and the breaker is placed inthe on position. With the breaker in this condition, the trip toggleadjusting screw 47, which is accessible through the central opening inthe top of the trip unit leasing, is given a final adjustment. Asmentioned above, a component of the main operating spring force tryingto move the carrier 31 upwardly, is transmitted, through theintermediate latch 34, to the toggle 4243. The proportion of such forcewhich is transmitted can be varied in a particular design in a number ofways, but most easily by varying the steepness of the latch angle of thecam surface 41.

It will also be observed that regardless of the latch angle, the amountof force required to force the trip toggle over-center can be varied byvarying the amount of over-set of the toggle. Thus the greater theover-set, the greater the force required to move the toggle back throughdead center, and vice versa. We have found that the best results areobtained as to stability, uniformity of calibration and dependability,when the over-set is assess? suchas. torequire aiforce'ofi%-lbi to 1 lb.force on the trip bar at a point adjacent the endof the calibratingscrew 52 to initiate movement and cause tripping. Following suchforce'calibration of the toggle, the adjusting screw 52'for the thermalelement 51 is adjusted in accordance with the predetermined currentconditions to cause thermal trippingasdesired.

As an independent adjustment, the adjusting screw 67 for the armaturebias spring may be adjusted to determine the maximum and minimum amountof current required to cause magnetic tripping. 3 By way of furtheradjustment, it is possible to replace the spring 60 with other springsof differing strength.

The trip unit 23 is also-provided with a bracket 86 (Figure 1) attachedthereto by suitable means such as by rivets and having an intermediateportion thereof spaced away from the top wall-of the trip unit toprovide a convenient means forlifting the-trip unit out of the circuitbreaker. The bracket 36 also includes a further offset portion 86A whichis adapted to extend within an opening 87 in the top wall ofthe circuitbreaker cover 11 to provide a marking or indication of the circuitbreaker current rating.

While we have shown one specific embodiment of our invention, it will beappreciated that many variations thereof may readily be made 'by thoseskilled in the art, and we therefore intend by-theappended claims tocover all such variations as fall within the true spirit and scope ofour invention.

What we claim as new and desire to secure by Letters Patent of theUnited States is:

1. An electric circuit breaker comprising a pair of relatively movablecontacts movable between open and closed circuit positions, atrip membermovable to cause automatic opening of said contacts, a magnetic armaturepivotally supported in said circuit breaker and including meansengageable withsaid trip member to cause tripping movement thereof uponpivotal movement of said armature in a first direction,-an insulatinghousing for said circuit breaker, an adjustinglever pivotally supportedin said housing and including a portion projecting outside of saidhousing for manual operation, and biasing spring means connected betweenan intermediate point of said adjusting lever and a point on saidarmature eccentric to the pivotal axis of said armature and adapted tobias said armature for pivotal movement in a direction opposite to saidfirst direction, said adjusting lever being movable along apredetermined path to move the line of action of said biasing springmeans relative to the pivotal axis of said armature whereby to varybiasing component of said biasing means.

2. An electric circuit breaker comprising a pair of relatively movablecontacts movable between open and closed circuit positions, a tripmember movable to cause automatic opening of said contacts, a magneticarmature pivotally supported in said circuit breaker and including meansengageable with said trip member to cause tripping movement thereof uponpivotal movement of said armature in a first direction, an insulatingenclosure, an adjusting lever pivotally supported in said enclosure,biasing spring means interconnected between an outer portion of saidadjusting lever and a point on said magnetic armature eccentric to thepivotal axis thereof to bias said armature for pivotal movement in adirection opposite to said first direction, said adju-sting'lever beingmovable about its pivotal axis to move the line of action of saidbiasing spring to alter the distance between the line of action of saidtension spring and the pivotal axis of said armature whereby to vary thebiasing component of said force, said enclosure having a plurality ofnotches in a wall thereof and means carried by said adjusting leveradapted to cooperate with said notches in said enclosure to provide aplurality of predetermined positions of said adjusting lever.

3.'An electric circuitbreaker comprising a pair of relatively movablecontactsmovable-between openand closed circuit positions, a trip membermovable to cause automatic opening of saidcontacts, a magnetic. armaturepivotally supported in said circuit-breaker and including meansengageable with said trip member to cause tripping movement thereof uponpivotal movement of said armature in a first direction, aninsulating-housing, an adjusting lever pivotally supported in saidinsulating housing, tension spring means interconnected between anintermediate point of said adjusting lever and a point on said armatureeccentric to'the'pivotal axis ofusaid armature to bias said armature forpivotal movement. in a direction opposite to said first direction, saidtension spring means being attached to said'adjusting lever by meanswhich is adjustable to vary .the length of said tension spring means.

4. An electric circuit breaker comprising a pair of relatively movablecontacts movable between openJand closed circuit positions, a tripmember movable to cause automatic opening of said contacts, amagnetic-armature pivotally supported in said circuit breaker andincluding means engageable with said trip member .to cause trippingmovement thereof upon pivotal movement of said armature in a firstdirection, aninsul-ating enclosure, an adjusting lever pivotallysupported in said enclosure, spring anchoring means carried by the outerportion of said adjusting lever, tension spring means connected betweensaid spring anchoring means and a point on said armature eccentric tothe pivotal axis of said armature to bias said armature for pivotalmovement in a direction opposite to said first direction, said springanchoring means being adjustable to vary the length of saidtensionspring means without movement of said adjusting .lever, and saidadjusting lever being movable to vary the distance between the lineofaction of said tension spring means and the pivotal axis of saidarmature whereby to vary the biasing component of said biasing springmeans substantially without change in the length of said spring.

5. An electric circuit breaker comprising a pair of relatively movablecontacts movable between open and closed circuit positions, a tripmember movable to cause automatic opening of said contacts, a magneticarmature pivotally supported in said circuit breaker and including meansengageable with said trip member to cause tripping movement thereofuponpivotal movement of said armature in a first direction, aninsulating enclosure, an adjusting lever pivotally supported in saidinsulating enclosure, a secondary adjusting lever carried by a portionof said adjusting lever remote from said pivotally supported portion,biasing spring means interconnected between a point on saidsecondary'adjusting lever and a point on said magnetic armatureeccentric to the pivotal axis thereof to bias said armature for pivotalmovement in a direction opposite to saidifirst direction, and means foradjusting the normal position ofsaid secondary adjusting lever on saidadjusting lever whereby to vary the force of said biasing spring meansWithout movement of said adjusting lever.

6. An electric circuit breaker comprising a pair of relatively movablecontacts, a member movable to cause automatic opening of said contacts,an insulating enclosure, an armature member pivotally mounted in saidinsulating enclosure, a first adjusting lever pivotally moimted in saidenclosure at one end thereof, spring means having one end thereofattached to said armature member and having the otherend thereofattached to said adjusting lever at a point remote from the pivotallysupported end, said enclosure having an elongatedslot therein, saidadjusting lever including a portion projecting through said elongatedslot and accessiblefor operation from outside said trip unit housing,said adjusting lever including an intermediate portion extendingsubstantially at right angles to the major portion thereof and saidenclosure including an'arcuate shield adjacent said elongated slotadapted to prevent direct access to the interior of said enclosurethrough said slot.

7. A multipole electric circuit breaker comprising a plurality of pairsof relatively movable contacts, a member movable to cause automaticopening of said contacts, an insulating base, an armature memberpivotally mounted on said base, an adjusting lever pivotally supportedon said insulating base at one end thereof, a tension spring having oneend thereof connected to said armature member and having the other endthereof connected to a point on said adjusting lever remote from saidpiv otally supported end, a common trip bar extending across all polesof said multipole circuit breaker, said armature member being pivotallysupported coaxially with said common trip bar, said common trip barincluding an intermediate portion having a notch therein extendingsubstantially to the pivotal axis thereof to permit movement of saidadjusting tension spring to a point where the line of action of saidspring extends substantially through the center of said axis of saidcommon trip bar.

8. An electric circuit breaker comprising an enclosure, a pair ofrelatively movable contacts movable between open and closed circuitpositions, a trip member movable to cause automatic opening of saidcontacts, a magnetic armature pivotally supported in said enclosure andineluding means engageable with said trip member to cause trippingmovement thereof upon pivotal movement of said armature in a firstdirection, an adjusting lever having a bearing portion at one endthereof, biasing spring means interconnected between a portion of saidadjusting lever remote from said bearing portion and a point on saidmagnetic armature eccentric to the pivotal axis of said armature to biassaid armature for pivotal movement in a direction opposite to said firstdirection, said enclosure including a bearing recess adapted to receivethe said bearing portion of said adjusting lever, said biasing springbeing adapted to hold said adjusting lever in said bearing recess.

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